This invention relates to ammunition rounds and more specifically to rounds which produce a low projectile velocity, as those rounds in which the projectile is a grenade. In any gun system which uses cartridge ammunition, the propellant is burned in a case behind the projectile and the hot gases generated thereby expand to propel the projectile from the case and along the gun bore. Where the projectile velocity is only a few to several hundred feet per second, it is generally impossible to obtain consistent muzzle velocities with conventional cartridges. This is because the rate of increase of the case volume is so rapid, as the projectile is propelled forwardly therein by the initial ignition, that a variable proportion of the propellant is driven along the gun bore after the projectile without being ignited. Thus, round to round combustion is inconsistent, resulting in a low reliability.